1. Field of the Invention
The present invention relates to an optical scanning apparatus employing a laser diode as a light source and an image forming apparatus employing such an optical scanning apparatus. The image forming apparatus may be used in digital copying machines, printers, facsimile devices, and other devices in which the image is formed by radiating a light laser beam onto a photosensitive body.
2. Discussion of the Background
Many image forming devices are provided for use in copying machines, printers, facsimiles, etc., in which a light beam output from a laser diode is radiated onto a photosensitive body, thereby forming an electrostatic latent image on the photosensitive body.
An example of such an image forming apparatus is found in the published specification of Japanese Laid-open Patent Publication No. 6-246974, which discloses the technology of measuring the temperature of a laser diode and compensating the optical output in accordance with the measured temperature, when the environmental temperature, etc., of the laser diode varies.
According to the above-mentioned Japanese Laid-open Patent Publication No. 6-246974, when the environmental temperature, etc., of the laser diode varies, the temperature of the laser diode is measured and the optical output is compensated in accordance with the measured temperature.
The above published specification further discloses that, when the temperature rises, the light emitting coefficient is lowered and the oscillation wavelength of the laser diode is increased. Furthermore, when the accumulative light emitting time is increased, light emitting efficiency is lowered. The operating temperature of the laser diode has to be equal to or lower than a certain temperature (preferably, below 60° C.). When the temperature exceeds the above-mentioned operating temperature, the laser diode breaks down.
Here, the actual temperature of a laser diode depends on the ambient temperature in the operating environment and the increase in temperature caused by heat emission of the laser diode itself. Regarding the amount of light emission from the laser diode itself, almost 95% of the input electric power is converted to thermal energy and the remaining part of the power is converted to laser light. In an image forming apparatus employing a laser diode, temperature near the optical scanning unit in the apparatus reaches almost 45° C. As a result, the increase in temperature caused by light emission from the laser diode itself is added to the temperature of the apparatus. For this reason, although it is not described in the above in specification, in the general prior art aluminum is used as the holding member for holding the laser diode. In such a structure, heat emission (radiation) is efficient.
The published specification discloses a method of holding the laser diode with a holding member made of metal material. The specification further discloses a fixing method for preventing the occurrence of the relative positional shift (difference) between the laser diode and a collimate lens caused by thermal expansion and contraction due to temperature variation.
Here, the laser diode performs heat emission at the time of emitting light where the amount of emitted heat corresponds to the thermal energy obtained by converting 95% of the input electric power to heat. Thus, light emission from a laser diode results in high-temperature heat emission. Laser diode functionality is considerably degraded and cannot be restored to its initial state when the temperature exceeds a certain temperature (e.g., 60° C.). For this reason, when a laser diode is used, it is generally required that the diode dissipate heat emitted from the laser diode itself A heat dissipating metal such as aluminum must be used, and therefore cost is inevitably increased.
On the other hand, in the recent years, in order to form an image with small pixel size and high pixel density (600 dpi or 1,200 dpi) in order to obtain a high-quality image, it is necessary to prepare an optical lens system for focusing laser diode light output onto a photosensitive body with a small spot diameter, and with high speed.
A method of enabling the output of an image with fine pixel density and high speed by constructing a light source with adjacently-arranged plural laser diodes and scanning the photosensitive body with plural light beam had been proposed previously. Another method uses a laser diode array (LDA) for outputting plural laser light beams. In this case, heat emission is greater than in the former method, and therefore, it is further important to effectively perform heat dissipation by the holding member of the laser diode.
On the other hand, in order to reduce the cost of the image forming apparatus, the cost of the respective parts has to be reduced. Therefore, it is desired to reduce the parts costs of the laser diode and the so-called LD holder for holding the laser diode.